Extendible and retractable hoists for raising and lowering the dump bed of a truck about a pivoting hinge are well known in the art and come in many different types and designs (whether they be rear or side dump vehicles). The term “truck” is used herein to describe all types of vehicles which employ such hoists, including light, medium and heavy duty dump trucks and trailers.
In a typical hydraulic truck hoist, one or more hydraulic cylinders in a hoist frame lie lengthwise in relation to the frame rails of the vehicle and underneath the truck bed. As the hoist's cylinder extends, one end of the hoist frame pushes against the underside of the dump bed to which one end of the hoist is pivotally attached while the other end of the hoist frame pushes against the frame of the truck to which the other end of the hoist is pivotally attached. This causes the bed to rotate about its rear hinge pivot whereby the dump bed is raised. Retraction of the cylinder causes the bed to be lowered.
Generally speaking, truck hoist designs known prior to the subject invention have one or more inefficiencies and/or drawbacks (or problems) associated with their use. For example, some hoist designs are configured to require long, telescopic hydraulic cylinder strokes to achieve sufficient dump angles. However, as a telescopic hydraulic cylinder extends e.g., in a long “stroke” towards its full displacement, the amount of force that it is capable of providing decreases significantly. The required length of the cylinder also impacts the cycle time required to operate the hoist. Although scissor-type hoists reduce this inefficiency (at least in part), scissor-type hoists exhibit other inefficiencies and/or problems.
For example, known scissor hoists are configured such that the hoist cylinder is nearly parallel to the dump bed when the dump bed is in the non-raised or lowered position and thus has very little initial leverage at the beginning of the lifting process. Specifically, in such designs, during the initial lift phase, much of the force of the cylinder is inefficiently directed because of the low angle of cylinder thrust relative to the truck frame and dump bed (which directs cylinder forces along the length of the truck and dump bed frame rather than in the more efficient raising direction which changes as the dump body is raised). Therefore, much of the hydraulic cylinder's lift force is unused at the beginning of lift operation. As a result, such prior art hoists often require larger and more expensive (or multiple) cylinders otherwise unnecessary in later lifting phases (i.e., they are only required to accommodate the initial, inefficient lifting phase).
In view of the above, it is apparent that there exists a need in the art for a hoist which addresses, overcomes, mitigates, and/or solves one or more of the above problems and/or drawbacks and/or inefficiencies in the art. It is a purpose of this invention to fulfill this and/or other needs in the art which will become more apparent to the skilled artisan once given the following disclosure.